Precious metal ores such as gold ores can be categorized as either free milling or refractory. Free milling ores are those that can be processed by simple gravity techniques or direct cyanidation. Refractory ores, on the other hand, are not amenable to conventional cyanidation treatment. Such ores are often refractory because of their excessive content of metallic sulfides such as pyrite or organic carbonaceous matter. A large number of refractory ores consist of ores with a precious metal such as gold occluded in iron sulfide particles. The iron sulfide particles consist principally of pyrite and arsenopyrite. If gold or other precious metals remain occluded within the sulfide host, even after grinding, then the sulfides must be oxidized to liberate the encapsulated precious metal values and make them amenable to a leaching agent.
Conventional biological methods have focused on the recovery of precious metals using sulfur-oxidizing bacteria. A conventional process includes the steps of distributing a concentrate of refractory sulfide minerals on top of a heap of material, biooxidizing the concentrate of refractory sulfide minerals, leaching precious metal values from the biooxidized refractory sulfide minerals with a lixiviant, and recovering precious metal values from the lixiviant.
Problems exist using sulfur-oxidizing organisms in bioleaching processes. These problems include nutrient access, air access, carbon dioxide access, the generation of sulfuric acid from reactions of the sulfur-oxidizing bacteria, and the generation of heat during the exothermic biooxidation reactions which can kill growing bacteria. Ores that are low in sulfide or pyrite, or ores that are high in acid consuming materials such as calcium carbonate or other carbonates, may also be problematic during heap biooxidation processes. The acid generated by these low pyrite ores is insufficient to maintain the low pH and high iron concentration needed for bacteria growth.
The bioremediation of various pollutants using butane-utilizing bacteria is disclosed in U.S. Pat. Nos. 5,888,396, 6,051,130, 6,110,372, 6,156,203, 6,210,579, 6,244,346 and 6,245,235, which are herein incorporated by reference.